When tuning turbos, we go down to 11.5 AFR at full boost.

Water/Methanol is the wrong way to go. Water/Methanol is used to allow aggressive timing under high-boost conditions, not try to make up for lean AFRs.

Also, how often do you plan on filling your water/methanol reservoir? And where do you plan on putting it?

What would be more effective here is some ECU tuning, if that's possible.